This invention relates to Portland cement and more particularly, to Portland cement mixes, which, when mixed with enough water to effect setting thereof, possess an adequate plastic period while achieving adequate and/or improved 1 and 28 day strengths.
Portland cement is a powdered material which is made by burning a ground mixture of limestone and clay or shale to produce clinkers composed primarily of mixed calcium silicates, calcium aluminates, and calcium aluminoferrites. The clinkers, together with a few percent of gypsum, are then ground to a fine powder, which, when mixed with water, forms a paste that, when properly made, sets within a few hours and hardens slowly.
When sand or sand, crushed rock and/or gravel are incorporated in cement paste, mortar and concrete are obtained. The paste acts as the cementing material and its composition has a decisive effect on the mechanical properties of the resultant concrete, i.e., strength and dimensional stability. During the early stages of hardening, while in contact with water or air saturated with water, concrete expands slightly. Subsequently, when the concrete is exposed to lower ambient humidities, it shrinks.
Standard, high-early-strength Portland cement compositions, i.e., mortars and concretes, develop strength very slowly and will on the average take about 6 hours to develop about 125 p.s.i. compressive strength and far in excess of 24 hours to develop about 5000 p.s.i. compressive strength. The time over which cement is permitted to set results in a steady increase in strength over a considerable period. Thus, standard procedures for testing cement strength properties include tests conducted after setting periods of 1,7 and 28 days. However, strength improvements in concrete continue for a number of years.
Frequently because the concrete is slow to gain strength after it begins to set and in order for the concrete to achieve an adequate 1-day strength, e.g., over about 5000 pounds per square inch (psi), an accelerated curing process must be employed. For example, in making concrete beams or pre-stressed concrete panels, it is desirable that the cast concrete object have adequate 1-day strength so that the mold or form can be removed from the object within a day or less. Then, the mold or form can be re-used to shape a further amount of concrete to make further panels or beams and so need not be tied up while the cast concrete object achieves the strength necessary to remove it from the shaping means, i.e., the mold form or the like. Frequently, cement products which achieve adequate 1-day strengths, when aided by an acceleration process, do not achieve adequate 28 day strengths, e.g., over 6000 psi. While this does not make the products inoperative, it does make them less than desirable.
One method by which cast concrete objects are cured in an accelerated fashion is that of steam curing. By that process, steam is played against the cast concrete objects, which have been placed in curing kilns to confine and concentrate the steam for a period of about 18 hours. While the products will achieve adequate one-day strength in this manner, the steam curing process involves a considerable expense in terms of combustion of fuel to fire boilers, distribution systems for the steam, curing kiln equipment, and the like, as well as the expense of maintenance.
If a Portland cement clinker is ground without the addition of a retarder (e.g., gypsum) its interaction with water is usually rapid causing a sharp increase in temperature termed a flash set. This is due to the rapid hydration of tricalcium aluminate accompanied by crystallization of the calcium aluminate hydrates that congeal the paste. The flash set which does result is not acceptable to produce a setting cement because set is so rapid as to prevent removal from the mixer and is not amenable to handling or finishing.
When gypsum is added as a retarder, it reacts rapidly with the dissolved aluminate in the presence of calcium hydroxide to form calcium sulfoaluminate. Even today, it is not quite clear why a small addition of gypsum is capable of preventing flash setting of cement. There is no doubt, however, that gypsum quickly reacts with tricalcium aluminate (C.sub.3 A) to form a compound practically insoluble in water; this compound is called calcium sulfoaluminate. The calcium sulfate or gypsum is added to the clinker usually in an amount of about 5 to 8% of the weight of the cement.
Even with the inclusion of the gypsum, early or premature stiffening (false set) sometimes occurs, which can be troublesome if it occurs before the concrete is placed. False set in cement is evidenced in concrete by a significant loss of consistency shortly after mixing. After additional remixing, however, this concrete regains its original consistency.
There are many additional problems arising from the use of gypsum. First, it is difficult to feed accurately, as large lumps are difficult to handle and fine materials favour segregation of the clinker-gypsum mixture. Second, gypsum has to be free from surface moisture and it has to be stored under shelter otherwise it tends to cake and bridge in the feed bins. Third, the quality of gypsum is not too consistent, since it contains such impurities as clay, silica and calcium carbonate. Fourth, gypsum (CaSO.sub.4. 2H.sub.2 O) often contains anhydrite (CaSO.sub.4) which has an unsatisfactory effect on the setting time of cement. Fifth, gypsum has no structural strength and weakens the strength of the Portland cement since as much as 8% gypsum is sometimes added to the clinker. Sixth, the temperature in the grinding mills has to be kept at less than 350.degree. F to prevent partial dehydration of gypsum to anhydrite.
Because of the unpredictable results which occur when using gypsum retarders, many efforts have been made to find substitutes for gypsum in order either to avoid having to use gypsum retarding mixes altogether, or alternatively to achieve more suitable mixes having more consistent performance characteristics. For example, Braunauer U.S. Pat. No. 3,689,294 adds an alkali lignosulfonate in combination with an alkali metal carbonate (in particular, potassium carbonate) to a finely ground (6000 to 9000 cm.sup.2 /g Blaine fineness) cement during the grinding step, and achieves a lower water-to-cement ratio than is conventionally utilized, as well as a workable concrete, while obviating the need to add gypsum to the cement powder. But Braunauer does so with a sacrifice of setting times, i.e., when Braunauer's 1-day strengths were sufficiently high in his mortars and concretes, the setting times were relatively low, e.g., less than 40 minutes, and in most instances less than 25 minutes. As another example, Landry, U.S. Pat. No. 3,689,296 teaches the use of a set retarder comprising the reaction product of formaldehyde and an aminolignosulfonate as a substitute for gypsum.
Thus, a need exists for an improved cement or concrete mix which provides an adequate plastic period, while at the same time providing an improved 1 and 28 day strength.